Image recording medium, image recording method and heat coloring polymer compound

ABSTRACT

An image recording medium is disclosed which comprising: an acid generating agent capable of generating an acid by the action of heat or an acid, which is represented by the following formula (1); and a compound of causing variation in the absorption region of from 360 to 900 nm by the intramolecular or intermolecular reaction triggered by the action of an acid:  
     W 1 OP   (1)  
     wherein W 1  represents a residue of an acid represented by W 1 OH, and P represents an acid-sensitive substituent capable of splitting off at a temperature of 150° C. or less due to catalysis by W 1 OH.

FIELD OF THE INVENTION

[0001] The present invention relates to an image recording medium usinga coloring (or decolorizing) reaction accelerated by an acid catalyst,more specifically, the present invention relates to an image recordingmedium having high sensitivity and good storability using an acidbreeding system.

[0002] The present invention also relates to a copolymer of a monomerhaving a partial structure capable of generating an acid by the heatingor action of an acid and a monomer having a partial structure of causingvariation in the absorption by the contact with an acid, to an imagerecording medium having high sensitivity and good storability using thepolymer and to an image forming method using the recording medium.

BACKGROUND OF THE INVENTION

[0003] The thermosensitive recording material expresses the image areaand the non-image area as a temperature difference distribution and manysystems therefor have been proposed, such as transfer by the melting orsublimation of a coloring agent, color formation reaction between twocomponents due to heat-melting or rupture of a capsule, or change in theoptical properties due to phase transition. These kinds ofthermo-sensitive recording mediums are advantageous in that a recordingimage can be obtained by a dry and simple system and the maintenance isnot necessary, thus, are widely used as an output material of variousprinters, word processors, facsimiles and the like. Furthermore, to keepup with the progress of laser recording devices in recent years,application of the thermosensitive recording medium to optical disks orphotomechanical materials is being studied.

[0004] The photomechanical material heretofore used is a silver halidelight-sensitive material requiring a wet processing. However, because ofdemands for simplification of the processing steps and problems ofenvironmental pollution by the processing solutions, development of adry process is required and several technical proposals using athermosensitive recording system have been recently made. In view of theresolution, recording of an image using a laser is preferred and forexample, a system using a high output laser, called dye ablation, hasbeen developed. Recording materials therefor are disclosed inJP-A-7-164755 (the term “JP-A” as used herein means an “unexaminedpublished Japanese patent application”), JP-A-7-149063 andJP-A-7-149065, and image forming apparatuses are disclosed inJP-A-8-48053 and JP-A-8-72400. According to this system, a recordingmaterial comprising a support having coated thereon a dye compositioncomprising an image dye, a substance having an absorption in the laserwavelength region (infrared ray absorptive substance) and a binder isirradiated with a laser beam from the dye layer side to record an image.The energy given by the laser beam brings about abrupt local changes inthe image forming layer at the spot of the laser beam thrusting thematerial and thereby expels the substance from the layer. According tothe above-described patent publications, this is not a completelyphysical change (for example, melting, evaporation or sublimation) but akind of chemical change (for example, bond destruction), and the imagedye is not partially removed but completely removed. The dye ablationsystem has, however, problems that a high output laser is indispensableso as to increase the dye removal efficiency at the site exposed to alaser beam and a dust collector for collecting dyes removed is necessaryto be installed in combination.

[0005] As a system which can dispense with a dust collector, an ablationtransfer image recording method using a laser heat source is disclosedin U.S. Pat. No. 5,171,650. This system uses a dye donor sheetcontaining a dynamic release layer formed by overcoating an ablativecarrier topcoat, where an image is transferred to a separate receivingsheet which is placed adjacently and aligned. Therefore, the sheet leftuseless after the recording of an image raises a problem of wastematerials. Furthermore, also in this case, a high output laser isindispensable so as to increase the transfer efficiency. Thus,conventional thermosensitive recording systems using ablation by a lasercannot dispense with a high output laser and are bound to the problem ofdusts or waste materials.

[0006] On the other hand, a method expanded from a system called “drysilver” is described in JP-A-6-194781 as a heat-recording systeminvolving no ablation using a laser. According to this system, recordingis performed by a laser on a recording material containing a silversource capable of thermal reduction, a reducing agent for silver ion anda light-heat conversion dye. This system is, however, deficient in thepractical performance in view of storability of the non-image area andheat sensitivity.

[0007] Another example of the thermosensitive recording system using alaser is a compound capable of changing the absorption due to thermaldecomposition of carbamate described in U.S. Pat. Nos. 4,602,263 and4,826,976. U.S. Pat. No. 5,243,052 describes a compound of forming ayellow color due to thermal decomposition of the t-butoxycarbonyl groupintroduced into the hydroxyl group. These systems use an irreversiblemonomolecular reaction and are suited for the image recording within ashort time using a laser, however, the sensitivity is not sufficientlyhigh and improvement for higher sensitivity is needed. Highersensitivity may be achieved by allowing an acid catalyst presenttogether but in turn a problem arises in the storage stability.

[0008] With respect to the method for forming a UV mask image (360 nm to420 nm; corresponding to the exposure light source for PS plate) usedfor the photomechanical material, heat mode systems using a laser havenot been proposed in practice.

[0009] U.S. Pat. Nos. 5,286,612, 5,395,736 and 5,441,850, WO94/10606 andWO94/10607 describe a recording material comprising a compound whichgenerates an extra strong acid of pKa<0 by the action of an ultravioletray, a secondary acid generating agent which generates a secondary acidin the presence of an extra strong acid, and a compound which changes inthe color upon contacting with the secondary acid. This system iseffective for achieving high sensitivity of a recording material.However, generation of the secondary acid from a secondary acidgenerating agent described in those patent publications is catalyzedonly by an extra strong acid generated from an extra strong acidprecursor and although the patent publications state that about 20molecules of the secondary acid precursor are catalyzed to decompose perone molecule of the extra strong acid, the secondary acid produced hasno catalytic capability. Accordingly, increase in the sensitivity by theacid breeding in geometrical progression cannot be expected.

[0010] As a compound which breeds an acid by the action of an acid, K.Ichimura, Chem. Lett., 551 (1995) and JP-A-8-248561 describe a compoundcapable of producing an organic acid due to decomposition by an acidcatalyst. In these publications, an acid reactive polymer compositioncomprising a combination of a photoacid generating agent and a substanceof causing structural change by the action of an acid is described.However, the structural change caused by the action of an acid generatedfrom a photoacid generating agent is to cause solubilization orinsolubilization in a polar solvent or an aqueous alkali solution andthe end use- thereof is a so-called photoresist. These publications arecompletely silent on the recording material using change in theabsorption.

[0011] The present inventors have made extensive investigations on thecompounds developed by Ichimura et al and have found that the compoundsreported by Ichimura et al mostly have a function as a thermal acidgenerating agent and can be used for the image formation because oftheir capability to act as an acid generating agent even if a photo orthermal acid generating agent is not newly added.

SUMMARY OF THE INVENTION

[0012] A first object of the present invention is to provide a novelimage recording medium which has high thermal sensitivity, enablesrecording at a low output laser on the level of not causing ablation(image recording responsive to 360 to 420 nm indispensable for thephotomechanical mask film) even when a heat mode image recording systemusing a laser is employed, can dispense with a separate image receivingsheet, and has excellent storage stability, and an image recordingmethod using the medium.

[0013] A second object of the present invention is to provide a novelpolymer capable of coloring (or decolorizing) only by an acid in acatalytic amount or by the action of heat.

[0014] These objects of the present invention can be attained by thefollowing inventions 1) to 27).

[0015] 1) An image recording medium comprising:

[0016] an acid generating agent capable-of generating an acid by theaction of heat or an acid, which is represented by the following formula(1); and

[0017] a compound of causing variation in the absorption region of from360 to 900 nm by the intramolecular or intermolecular reaction triggeredby the action of an acid:

W¹Op   (1)

[0018] wherein W¹ represents a residue of an acid represented by W¹OH,and P represents an acid-sensitive substituent capable of splitting offat a temperature of 150° C. or less due to catalysis by W¹OH.

[0019] 2) The image recording medium as described in 1), wherein theacid generating agent represented by formula (1) is a compound selectedfrom the compounds represented by the following formulae (2) to (5):

[0020] wherein R¹ represents an alkyl group or an aryl group; R²represents an alkyl group; R³ represents a secondary or tertiary alkylgroup having a hydrogen atom at the β-position; and W¹ represents aresidue of an acid represented by W¹OH;

[0021] wherein R⁴, R⁵, R⁶ and R⁹ each represents a hydrogen atom, analkyl group or an aryl group; R⁷ and R⁸ each represents an alkyl group,an aryl group or a silyl group; R⁷ and R⁸ may form a ring; X representsO or S; and W¹ represents a residue of an acid represented by W¹OH;

[0022] wherein R¹⁰, R¹¹ and R¹² each represents a hydrogen atom, analkyl group or an aryl group; and W¹ represents a residue of an acidrepresented by W¹OH;

[0023] wherein R¹³ represents an alkyl group or an aryl group; and W¹represents a residue of an acid represented by W¹OH.

[0024] 3) The image recording medium as described in 1) or 2), whichfurther comprises a compound capable of generating an acid by the actionof light or heat in addition to the acid generating agent represented byformula (1).

[0025] 4) The image -recording medium as described in 3) wherein theacid generating agent capable of generating an acid by the action oflight or heat is a compound capable of generating a sulfonic acid, acarboxylic acid or a phosphoric acid.

[0026] 5) The image recording medium as described in 1) to 4), whereinthe compound of causing variation in the absorption region of from 360to 900 nm by the action of an acid is a compound having at least oneamino group substituted by a substituent which is accelerated to splitoff by the action of an acid, and causing variation in the absorptionregion due to the splitting off of said substituent.

[0027] 6) The image recording medium as described in 5), wherein saidsubstituent of the amino group, which is accelerated to split off by theaction of an acid, is a secondary or tertiary alkoxycarbonyl grouphaving a hydrogen atom at the β-position.

[0028] 7) The image recording medium as described in 1) to 4), whereinthe compound of causing variation in the absorption region of from 360to 900 nm by the action of an acid is a compound having at least onehydroxyl group substituted by a substituent which is accelerated tosplit off by the action of an acid, and causing variation in theabsorption region due to removal of said substituent.

[0029] 8) The image recording medium as described in 7), wherein thesubstituent of the hydroxyl group, which is accelerated to split off bythe action of an acid, is a secondary or tertiary alkoxycarbonyl grouphaving a hydrogen atom at the β-position, an alkoxymethyl group or asilyl group.

[0030] 9) The image recording medium as described in 1) to 8), whichfurther comprises an infrared ray absorptive substance.

[0031] 10) A method for recording an image, which comprisesscan-exposing the image recording medium described in 1) by a laser beamand then heating the entire surface of the image recording medium at atemperature of from 60° to 150° C.

[0032] 11) A polymer represented by the following formula (6):

[0033] wherein A represents a repeating unit obtained by polymerizing atleast one vinyl monomer having a function of generating an acid by theaction of an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.

[0034] 12) The polymer as described in 11), wherein A in formula (6) hasa function of generating an acid by the action of heat.

[0035] 13) The polymer as described in 12), wherein A in formula (6) isa repeating unit obtained by polymerizing at least one vinyl monomerrepresented by the following formula (7):

[0036] wherein R²¹ represents an electron-withdrawing group having aHammett's up value greater than 0; R²² represents an alkyl group or anaryl group; R²³ represents a secondary or tertiary alkyl group having ahydrogen atom at the β-position; W² represents a residue of an acidrepresented by W²OH; and any one of the substituents R²¹, R²², R²³ andW² represents a polymerizable vinyl group.

[0037] 14) An image recording medium which comprises a polymer havingboth (i) a partial structure of generating an acid by the action of anacid and further breading an acid by the action of an acid generatedfrom the partial structure itself and (ii) a partial structure ofcausing variation in the absorption region of from 360 to 900 nm by theaction of an acid.

[0038] 15) The image -recording medium as described in 14), wherein thepolymer is a polymer represented by formula (6):

[0039] wherein A represents a repeating unit obtained by polymerizing atleast one vinyl monomer having a function of generating an acid by theaction of an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.

[0040] 16) The image recording medium as described in 14), which furthercomprises an acid generating agent capable of generating an acid by theaction of light or heat.

[0041] 17) The image recording medium as described in 14), which furthercomprises (i) an acid generating agent capable of generating an acid bythe action of heat and (ii) an infrared ray absorptive substance.

[0042] 18) The image recording medium as described in 14), wherein thepolymer is a polymer represented by formula (6), and the image recordingmedium further comprises an infrared ray absorptive substance, andwherein A in formula (6) has a function of generating an acid by theaction of heat.

[0043] 19) A method for recording an image, which comprisesscan-exposing the image recording medium described in 14) by a laserbeam.

[0044] 20) A method for recording an image, which comprisesscan-exposing an image recording medium described in 14) by a laser beamand then heating the entire surface of the image recording medium at atemperature of from 60° to 150° C.

[0045] 21) An image recording medium comprising a polymer having both amoiety of generating an acid by the action of an acid and a partialstructure of causing variation in the absorption region of from 360 to900 nm by the action of an acid.

[0046] 22) The image recording medium as described in 21), which furthercomprises an acid generating agent capable of generating an acid by theaction of light or heat.

[0047] 23) The image recording medium as described in 21), wherein thepolymer is a polymer represented by formula (6).

[0048] 24) The image recording medium as described in 21), wherein A informula (6) is a repeating unit obtained by polymerizing at least onevinyl monomer represented by formula (7).

[0049] 25) The image recording medium as described in 21), which furthercomprises an infrared ray absorbing substance.

[0050] 26) A method for forming an image, which comprises scan-exposingthe image recording medium described in 21) by a laser beam.

[0051] 27) A method for forming an image, which comprises scan-exposingthe image recording medium described in 21) by a laser beam and thenheating the entire surface of the image recording medium at atemperature of from 60° to 150° C.

DETAILED DESCRIPTION OF THE INVENTION

[0052] The present invention is described in detail below.

[0053] The compound represented by formula (1) has a function ofgenerating an acid self-breedingly by the action of an acid generatedfrom the acid generating agent itself due to thermal decomposition, or afunction of generating an acid self-breedingly by the action of an acidgenerated from the acid generating agent itself after an acid isgenerated by the reaction of the acid generating agent and an acidgenerated from the other acid generating agent.

[0054] In the acid generating agent represented by formula (1), W¹represents a residue of an acid represented by W¹OH (e.g., sulfonicacid, carboxylic acid, phosphoric acid, phenol). W¹OH is preferably anacid having pKa<3. Preferred examples thereof include p-toluenesulfonicacid, benzenesulfonic acid, methanesulfonic acid,polyvinyl-benzenesulfonic acid and p-nitrobenzoic acid.

[0055] P represents an acid-sensitive substituent which can be removedby the action of an acid. With the removal of P, an acid represented byW¹OH is formed from an acid generating agent represented by W¹OP.Furthermore, P is a substituent capable of splitting off at atemperature of 150° C. or less using the acid (W¹OH) formed by thedecomposition of the acid generating agent (W¹OP) as a catalyst. Asubstituent capable of splitting off at a temperature of 120° C. or lesswith a catalyst of W¹OH is preferred. The term “catalyze or catalysis”as used in the present invention means that the decompositiontemperature lowers by 10° C. or more, preferably 20° C. or more byadding an acid as compared with the case of using the compound alone.Examples of such a protective group include an alkyl group having ahydrogen atom at the β-position (e.g., tetrahydropyranyl,tetrahydrofuranyl, t-butyl, cyclohexyl, 4,5-dihydro-2-methylfuran-5-yl,2-cyclohexenyl), an alkoxycarbonyl group having a hydrogen atom at theβ-position (e.g., t-butoxycarbonyl, cyclohexyloxycarbonyl,2-(2-methyl)-butoxycarbonyl, 2-(2-phenyl)propyloxycarbonyl,2-chloro-ethoxycarbonyl), a silyl group (e.g., trimethylsilyl,triethylsilyl, t-butyldimethylsilyl, phenyldimethylsilyl), and aprotective group which is triggered to split off by the decomposition ofthe above-described group or acetal, ketal, thioketal, pinacol or epoxyring thereof (examples of this protective group include the groupssubstituted by W¹OH described later in the description of formulae (1)to (4)).

[0056] Examples of the self-breeding type acid generating agentrepresented by formula (1) include trifluoroacetic acid(α-phenyl-isopropyl) ester, trifluoroacetic acid t-butyl ester,toluenesulfonic acid cyclohexyl ester, triethylsilyl p-nitrobenzoate,tetrahydropyranyl p-nitrobenzoate,poly(4-vinyl-1-t-butoxycarbonyloxy-2-nitrobenzene) and poly(cyclohexyl4-vinylbenzenesulfonate), however, in view of the breeding sensitivityand storage stability, the following compounds are particularlypreferred in the present invention.

[0057] A first example is a compound represented by formula (2).

[0058] In formula (2), R¹ represents an alkyl group (inclusive of analkyl group having a substituent, preferably having from 1 to 60 carbonatoms, e.g., methyl, ethyl, iso-propyl, t-butyl, trifluoromethyl,ethoxymethyl) or an aryl group (inclusive of an aryl group having asubstituent, preferably having from 6 to 60 carbon atoms, e.g., phenyl,naphthyl, 4-chlorophenyl, 2-methoxyphenyl, 4-nitrophenyl,3-methanesulfonylphenyl), R² represents an alkyl group (preferably analkyl group described for R¹), R³ represents a secondary or tertiaryalkyl group having a hydrogen atom at the β-position (inclusive of onehaving a substituent, preferably having from 3 to 60 carbon atoms, e.g.,t-butyl, cyclohexyl, tetrahydropyranyl, tetrahydrofuranyl,4,5-dihydro-2-methylfuran-5-yl, 2-cyclohexenyl), and W¹ represents aresidue of an acid represented by W¹OH (W¹OH preferably has a pKa of 3or less, e.g., p-toluenesulfonic acid, benzenesulfonic acid,methanesulfonic acid, polyvinylbenzenesulfonic acid, p-nitrobenzoicacid).

[0059] The self-breeding type acid generating agent represented byformula (2) may be formed as a polymer by connecting a plurality ofpolymerizable groups introduced into the sites capable of substitution.In this case, the polymer may be either a homopolymer or a copolymerwith another monomer. The polymer preferably has a molecular weight offrom 1,000 to 1,000,000, more preferably from 2,000 to 100,000.

[0060] Specific examples of the compound represented by formula (2) areset forth below, however, the present invention is by no means limitedthereto.

[0061] A second example is a compound represented by formula (3).

[0062] In formula (3), R⁴, R⁵, R⁶ and R⁹ each represents a hydrogenatom, an alkyl group (preferred examples thereof are the same as thosedescribed for R¹) or an aryl group (preferred examples thereof are thesame as those described for R¹), R⁷ and R⁸ each represents an alkylgroup (preferred examples thereof are the same as those described forR¹), an aryl group (preferred examples thereof are the same as thosedescribed for R¹) or a silyl group (e.g., trimethylsilyl, triethylsilyl,t-butyldimethyl, phenyldimethylsilyl), R⁷ and R⁸ may be combinedtogether to form a ring and in the case when R⁷ and R⁸ form a ring, thering is preferably a 5-, 6-, 7-, 8-, 9- or 10-membered ring, preferablya 5- or 6-membered ring, formed together with —O—C—X—, X represents O orS, and W¹ represents a residue of an acid represented by W¹OH (preferredexamples thereof are the same as those described for R¹).

[0063] The self-breeding type acid generating agent represented byformula (3) may be formed as a polymer by connecting a plurality ofpolymerizable groups introduced into the sites capable of substitution.In this case, the polymer may be either a homopolymer or a copolymerwith another monomer. The polymer preferably has a molecular weight offrom 1,000 to 1,000,000, more preferably from 2,000 to 100,000.

[0064] Specific examples of the compound represented by formula (3) areset forth below, however, the present invention is by no means limitedthereto.

[0065] A third example is a compound represented by formula (4).

[0066] In formula (4), R¹⁰, R¹¹ and R¹² each represents a hydrogen atom,an alkyl group (preferred examples thereof are the same as thosedescribed for R¹) or an aryl group (preferred examples thereof are thesame as those described for R¹), W¹ represents a residue of an acidrepresented by W¹OH (preferred examples thereof are the same as thosedescribed for R¹).

[0067] The self-breeding type acid generating agent represented byformula (4) may be formed as a polymer by connecting a plurality ofpolymerizable groups introduced into the sites capable of substitution.In this case, the polymer may be either a homopolymer or a copolymerwith another monomer. The polymer preferably has a molecular weight offrom 1,000 to 1,000,000, more preferably from 2,000 to 100,000.

[0068] Specific examples of the compound represented by formula (4) areset forth below, however, the present invention is by no means limitedthereto.

[0069] A fourth example is a compound represented by formula (5).

[0070] In formula (5), R¹³ represents an alkyl group (preferred examplesthereof are the same as those described for R¹) or an aryl group(preferred examples thereof are the same as those described for R¹), W¹represents a residue of an acid represented by W¹OH (preferred examplesthereof are the same as those described for R¹).

[0071] The self-breeding type acid generating agent represented byformula (5) may be formed as a polymer by connecting a plurality ofpolymerizable groups introduced into the sites capable of substitution.In this case, the polymer may be either a homopolymer or a copolymerwith another monomer. The polymer preferably has a molecular weight offrom 1,000 to 1,000,000, more preferably from 2,000 to 100,000.

[0072] Specific examples of the compound represented by formula (5) areset forth below, however, the present invention is by no means limitedthereto.

[0073] The compounds represented by formulae (1) to (4) can besynthesized according to the method described in JP-A-8-248561.

[0074] The amount of the self-breeding acid generating agent addedvaries depending on the kind of the compound of causing change in theabsorption, however, it is preferably from 0.001 to 20 equivalents, morepreferably from 0.01 to 5 equivalents, to the compound of causing changein the absorption.

[0075] As the compound which newly generates an acid due to the acidgenerated from the acid generating agent, U.S. Pat. Nos. 5,286,612,5,395,736 and 5,441,850, WO94/10606 and WO94/10607 describe a squarylicacid derivative and an oxalic acid derivative. These compounds are,however, catalyzed to decompose by an extra acid (acid having pKa<0) andaccordingly, the secondary acid produced by the decomposition cannotsubstantially catalyze further decomposition of a secondary acidprecursor. On the other hand, in the present invention, an acidgenerating agent such that an acid is increased by self-breeding it byutilizing an acid generated from the acid generating agent itself isused and thereby, the sensitivity can be greatly improved. Thisdecomposition is of course catalyzed by an extra acid but can becatalyzed by an acid on the order of a sulfonic acid, a carboxylic acidand a phosphoric acid and in the present invention, an acid generatingagent capable of generating a sulfonic acid, a carboxylic acid or aphosphoric acid is preferably used in view of storability.

[0076] In the case that the image recording medium of the presentinvention is used in a photon mode image recording system, it isnecessary to add a compound which generates an acid by the action oflight.

[0077] In contrast, in the case that the image recording medium of thepresent invention is used in a heat mode image recording system, whileit is not necessary to additionally add a thermal acid generating agentif the acid generating agent represented by formula (1) also has afunction of generating an acid by the action of heat since an acid isgenerated by the partial decomposition by the action of heat due to alaser irradiation, a thermal acid generating agent may be addedadditionally for the purpose of attaining high sensitivity. Furthermore,in the case that the acid generating agent represented by formula (1)does not have a function of generating a acid by the action of heat, itis necessary to add a thermal acid generating agent additionally.

[0078] The acid generating agent capable of generating an acid by theaction of light or heat is a compound which is originally not an acidbut decomposes upon irradiation of light or under heating and generatesan acid. In the present invention, an acid generating agent which onlydecomposes upon laser exposure but does not decompose during storage orat the heating of the entire surface after the laser exposure ispreferred.

[0079] With respect to the compound capable of generating an acid by theaction of light, various examples (e.g., nonionic compounds such as ahalide capable of generating hydrogen halogenide, a sulfide capable ofgenerating a sulfonic acid, a carbonylated capable of generating acarboxylic acid and a phosphorus compound capable of generating aphosphoric acid; ionic compounds such as various onium salts) aredescribed in Imaging yo Yuki Zairyo (Organic Materials for Imaging), pp.187-198, compiled by Yuki Electronics Zairyo Kenkyu Kai, published byBunshin Shuppan (1993). Any of these can be used in the presentinvention, however, in view of low toxicity, acid generating agentscapable of a sulfonic acid, a carboxylic acid or a phosphoric acid areparticularly preferred. For the purpose of widening the sensitivityregion of the photoacid generating agent, various sensitizers (examplesthereof are described in J. Polymer Sci., 16 2441 (1978) and the like)can be added.

[0080] Specific examples of the compound capable of generating an acidby the action of light, which can be effectively used in the presentinvention, are set forth below, however, the present invention is by nomeans limited thereto.

[0081] A first example of the thermal acid generating agent capable ofgenerating an acid by the action of heat includes a sulfonate, a PF₆salt, a AsF₆ salt and a SbF₆ salt of an aromatic onium compound such asdiazonium, iodonium, sulfnium and phosphonium. These are a compoundwhich generates an acid also by the action of light and examples thereofinclude those described as examples of the photoacid generating agent.

[0082] A second example of the thermal acid generating agent includes anammonium salt and an amine complex of an acid. The ammonium salt of anacid can be synthesized by neutralizing an acid with ammonia or amine.Examples of the acid include p-toluenesulfonic acid, benzenesulfonicacid, 1,8-naphthalenedisulfonic acid, p-nitrobenzoic acid,2,4-dichlorobenzoic acid, phenylphosphonic acid and HPF₆. Examples ofthe amine complex of an acid include complexes obtained by mixingBF₃Et₂O with an amine. The amine may be any of primary amine, secondaryamine and tertiary amine and preferably has good volatility. Examplesthereof include methylamine, ethylamine, iso-propylamine, t-butylamine,aniline and pyridine.

[0083] Specific examples of the ammonium salt of an acid are set forthbelow, however, the present invention is by no means limited thereto.

[0084] A third example of the thermal acid generating agent includes acompound which causes β-hydrogen elimination by the heating to generatean acid. Examples of this compound include an alkyl ester having ahydrogen atom at the β-position of a sulfonic acid, a carboxylic acid ora phosphoric acid (for example, t-butyl ester, cyclohexyl ester,2-phenyl ethyl ester, 4,5-dihydro-2-methylfuran-5-yl ester and2-cyclohexenyl ester) and a compound represented by the followingformula (9):

YO—N═CH—Z   (9)

[0085] wherein Y represents a residue of a sulfonic acid, a carboxylicacid or a phosphoric acid represented by YOH, and Z represents asubstituted or unsubstituted aryl group preferably having from 6 to 60carbon atoms, such as a phenyl group, a 2,4-dimethoxyphenyl group, a1-naphthyl group, a 2-methoxy-1-naphthyl group and a 2-chloro-1-naphthylgroup.

[0086] Specific examples of the compound which causes β-hydrogenelimination by the heating to generate an acid are set forth below,however, the present invention is by no means limited thereto.

[0087] A fourth example of the thermal acid generating agent includes a2-nitrobenzyl ester and a benzoin ester of a sulfonic acid, a carboxylicacid or a phosphoric acid. These are a compound which generates an acidalso by the action of light, and examples thereof include thosedescribed above as examples of the photoacid generating agent.

[0088] A fifth example of the thermal generating agent includes acompound having a structure of an acid generating agent represented byformula (2). Preferred examples thereof are the same as those describedabove as examples of the self-breeding type acid generating agent.

[0089] Any of these thermal acid generating agents can be used in thepresent invention, however, in view of storage stability and lowtoxicity, acid generating agents which generate a sulfonic acid, acarboxylic acid or a phosphoric acid are particularly preferred. Thedecomposition temperature is preferably from 130° to 300° C., morepreferably from 150° to 250° C.

[0090] By controlling the amount of these acid generating agent added,the sensitivity and the storability of recording material can becontrolled. In general, the acid generating gent is preferably added inan amount of from 1 to 50 mol %, more preferably from 5 to 20 mol %,based on the self-breeding type acid generating agent represented byformula (1) which is described above.

[0091] The compound of causing change in the absorption region of from360 to 900 nm due to intramolecular or intermolecular reaction provokedby the action of an acid, for use in the present invention is a compoundwhich is stable as long as it is stored under the neutral to basiccondition but when applied by the action of an acid, is reduced in theactivation energy of the intramolecular or intermolecular reaction toallow the reaction to readily proceed by the heating and as a result,causes change in the absorption. In this case, the heating temperaturefor forming an image is preferably from 60° to 150° C., more preferablyfrom 80° to 120° C.

[0092] The compound of causing change in the absorption may be a singlecompound or may comprise two or more components. Examples thereofinclude a compound which forms a decolorized image in theabove-described region by the Diels-Alder reaction (e.g.,9,10-distyrylanthracene with maleic anhydride,tetra-phenylcyclopentadiene with acrylic acid ester), a compound whichforms a colored image in the above-described region by the retroDiels-Alder reaction (e.g., adduct of 9,10-distyrylanthracene and maleicanhydride, adduct of diphenylisobenzofuran and acrylamide), a compoundwhich forms a colored image in the above-described region as a result ofexpansion of the conjugated system by the β-hydrogen elimination (e.g.,1-acetoxy-1,2-diarylethane, 1-sulfoxy-1,2-diarylethane), a combinationof aldehyde which forms a colored image in the above-described region bythe dehydration condensation with an active methylene compound (e.g.,photographic 4-equivalent magenta coupler with p-metoxycinnamaldehyde)and a compound having an amino group or hydroxyl group substituted by asubstituent accelerated to decompose or split off by the action of anacid and as a result of elimination of the substituent, causing changesin the absorption in the above-described absorption region. Furthermorea basic leuco dye and the like which instantly forms a color uponcontact with an acid may be used in the image forming medium, however,since such a compound acts as a base and inhibits the process of acidbreeding, the compound must be isolated from the acid generating agentand the acid breeding agent by using a microcapsule or by coating thecomposition on a separate layer.

[0093] In the present invention, a compound of causing change in theabsorption as a result of decomposition or splitting off of thesubstituent of an amino group or hydroxyl group by the action of an acidis particularly useful.

[0094] Preferred examples of the substituent of the amino group includean alkoxycarbonyl group (e.g., t-butoxycarbonyl, cyclohexyloxycarbonyl,2-(2-methyl)butoxycarbonyl, 2-(2-phenyl)propyloxycarbonyl,2-chloroethoxycarbonyl), an acyl group (e.g., acetyl, benzoyl,2-nitrobenzoyl, 4-chlorobenzoyl, 1-naphthoyl) and a formyl group. Ofthese, an alkoxycarbonyl group having a hydrogen atom at the β-positionis more preferred in the present invention in view of storage stabilityand heat sensitivity. Such a compound is described, for example, in U.S.Pat. Nos. 4,602,263 and 4,826,976. By combining this compound with thethermal acid generating agent and acid breeding agent of the presentinvention, a thermosensitive recording material having highersensitivity and excellent storability can be obtained.

[0095] Specific examples of the compound having an amino groupsubstituted by the above-described substituent, which is useful in thepresent invention, are set forth below, however, the present inventionis by no means limited thereto.

[0096] Preferred examples of the substituent of a hydroxyl group, whichdecomposes or splits off by the action of an acid, include a secondaryor tertiary alkoxycarbonyl group having a hydrogen atom at theβ-position (e.g., t-butoxycarbonyl, isopropyloxycarbonyl,1-phenylethoxycarbonyl, 1,1-diphenylethoxycarbonyl,2-cyclohexeneoxycarbonyl), a silyl group (e.g., trimethylsilyl,triethylsilyl, t-butyldimethylsilyl, t-butyldiphenylsilyl,phenyldimethylsilyl), an alkoxymethyl group (e.g., methoxymethyl,ethoxymethyl, 1-methoxyethyl, 1-phenoxyethyl, 2-(2-metoxypropyl)) andsecondary or tertiary alkyl group having a hydrogen atom at theβ-position (e.g., tetrahydropyranyl, tetrahydrofuranyl,4,5-dihydro-2-methylfuran-5-yl, t-butyl, 2-cyclohexenyl). Of these, asecondary or tertiary alkoxycarbonyl group having a hydrogen atom at theβ-position is more preferred in the present invention.

[0097] Examples of the compound of causing change in the absorption dueto decomposition of the substituent of a hydroxyl group are described inU.S. Pat. No. 5,243,052 and JP-A-9-25360.

[0098] Specific examples of the compound of causing change in theabsorption in the above-described region due to decomposition of thesubstituent of a hydroxyl group, which is useful in the presentinvention, are set forth below, however, the present invention is by nomeans limited thereto.

[0099] The image recording medium of the present invention is generallymanufactured by coating the above-described acid generating agent, acidbreeding agent and composition of causing change in the absorptionregion of from 360 to 900 nm by the action of an acid on a support. Atthis time, a binder is usually used together except for the case whereany of these members is a polymer or an amorphous substance having goodcoatability. The case where a binder can be dispensed with isadvantageous in that the film thickness can be easily reduced and asharp image can be obtained.

[0100] In the case of using a binder, either a water-soluble binder suchas gelatin, casein, starches, hydroxyethyl cellulose, carboxymethylcellulose, polyvinyl alcohol, polyacrylamide and ethylene-maleic acidcopolymer, or a water-insoluble binder such as polyvinylbutyral,triacetyl cellulose, polystyrene, methyl acrylate-butadiene copolymer,acrylonitrile-butadiene copolymer may be used.

[0101] In formula (6), A represents a repeating unit obtained bypolymerizing at least one vinyl monomer having a function of generatingan acid by the action of an acid and further breeding an acid by theaction of an acid generated from the monomer itself or a function ofgenerating an acid by heat. Examples of the compound having such afunction include compounds described in Chemistry Letter, p. 551 (1995)and JP-A-8-248561 and cyclohexyl sulfonate.

[0102] The polymer having a thermal acid generating function of thepresent invention is advantageous in that when it is applied to an imagerecording medium in a laser heat mode system, another acid generatingagent is not necessary to be added.

[0103] The preferred embodiment of A having such a function includes thestructure represented by formula (7).

[0104] In formula (7), R²¹ represents an electron-withdrawing grouphaving a Hammet's σp value greater than 0 (having preferably from 0 to60 carbon atoms, for example, an acyl group such as acetyl and benzoyl,a sulfonyl group such as methanesulfonyl and benzenesulfonyl, aheterocyclic residue such as benzothiazolyl and benzoxazolyl, and atrifluoromethyl group), R² represents an alkyl group (inclusive of analkyl group having a substituent, preferably having from 1 to 60 carbonatoms, e.g., methyl, ethyl, iso-propyl, t-butyl, trifluoromethyl,ethoxymethyl) or an aryl group (inclusive of an aryl group having asubstituent, preferably having from 6 to 60 carbon atoms, e.g., phenyl,naphthyl, 4-chlorophenyl, 2-methoxyphenyl, 4-nitrophenyl,3-methanesulfonylphenyl), R³ represents a secondary or tertiary alkylgroup having a hydrogen atom at the β-position (inclusive of an alkylgroup having a substituent, preferably having from 3 to 60 carbon atoms,e.g., t-butyl, cyclohexyl, tetrahydropyranyl, tetrahydrofuranyl,4,5-dihydro-2-methylfuran-5-yl, 2-cyclohexenyl), W² represents a residueof an acid represented by W²OH (W²OH preferably having a pKa of 3 orless, e.g., p-toluenesulfonic acid, benzenesulfonic acid,benzenesulfonic acid, methanesulfonic acid, polyvinylbenzenesulfonicacid, p-nitrobenzoic acid), and at least one of R²¹, R²², R²³ and W²contains a polymerizable vinyl group (for example, groups shown below)at the site capable of substitution.

[0105] Specific examples of the vinyl monomer for forming A of formula(6) are set forth below, however, the present invention is by no meanslimited thereto. These compounds can be synthesized in the same manneras in the method described in JP-A-8-248561.

[0106] In formula (6), B represents a repeating unit obtained by thepolymerization of at least one vinyl monomer having a partial structureof causing change in the absorption region of from 360 to 900 nm by theaction of an acid. The partial structure of causing change in thisabsorption region by the action of an acid is described above.

[0107] The monomer for forming a repeating unit represented by B informula (6) by the polymerization is obtained by introducing apolymerizable vinyl group (examples thereof include those describedabove for A) into the site capable of substitution in theabove-described partial structure of causing change in the absorption bythe action of an acid. Examples thereof are set forth below, however,the present invention is by no means limited thereto.

[0108] C in formula (6) represents a repeating unit obtained bypolymerizing at least one vinyl monomer capable of forming a copolymertogether with A and B, and the storage stability, coloring activity andthe like can be controlled by adjusting the polarity, glass transitiontemperature or the like. Preferred examples thereof include acrylicester, methacrylic ester, acrylamide, styrene and vinyl ether. Specificexamples of the monomer for forming C are set forth below, however, thepresent invention is by no means limited thereto.

[0109] In formula (6), x, y and z represent % by weight of respectivecompositions. x, y and z satisfy the conditions of 1≦x≦99, 1≦y≦99,0≦z≦98 and x+y+z=100. x and y are preferably in the relation of0.01y≦x≦10y, more preferably 0.1y≦x≦5y. z preferably satisfies thecondition of 0≦z≦50.

[0110] The polymer represented by formula (6) preferably has a molecularweight of from 1,000 to 1,000,000, more preferably from 2,000 to100,000. The polymer may have any form of a random copolymer, analternate copolymer and a block copolymer, however, a random copolymereasy to synthesize is commonly used.

[0111] The monomers for forming A, B and C and the preferredcombinations of x, y and z in the polymer represented by formula (6) areshown below, however, the present invention is by no means limitedthereto. TABLE 1′ A B C x y z P-(1) A-(1) B-(1) — 67 33 — P-(2) A-(1)B-(1) — 75 25 — P-(3) A-(1) B-(1) C-(17) 50 25 25 P-(4) A-(1) B-(1)C-(18) 50 25 25 P-(5) A-(1) B-(1) C-(1) 50 25 25 P-(6) A-(1) B-(1)C-(11) 40 20 40 P-(7) A-(1) B-(5) — 67 33 — P-(8) A-(1) B-(6) — 67 33 —P-(9) A-(1) B-(13) — 67 33 — P-(10) A-(1) B-(19) — 67 33 — P-(11) A-(1)B-(21) — 67 33 — P-(12) A-(1) B-(25) — 67 33 — P-(13) A-(1) B-(29) — 6733 — P-(14) A-(1) B-(41) — 67 33 — P-(15) A-(1) B-(41) — 75 25 — P-(16)A-(1) B-(41) — 80 20 — P-(17) A-(1) B-(43) — 75 25 — P-(20) A-(2) B-(1)— 67 33 — P-(21) A-(6) B-(1) — 67 33 — P-(22) A-(8) B-(1) — 67 33 —P-(23) A-(13) B-(1) — 67 33 — P-(24) A-(28) B-(5) C-(10) 50 10 40 P-(25)A-(30) B-(41) C-(14) 60 20 20 P-(26) A-(41) B-(1) — 67 33 — P-(27)A-(42) B-(41) — 67 33 — P-(28) A-(43) B-(26) — 67 33 — P-(29) A-(45)B-(1) — 67 33 — P-(30) A-(47) B-(1) — 67 33 — P-(31) A-(1) B-(1) — 50 50— P-(32) A-(1) B-(1) — 20 80 — P-(33) A-(1) B-(1)/B-(28)/B-(42) C-(1) 4040 10 (= 1/1/1) P-(34) A-(1) B-(1)/B-(28) (= 1/1) — 50 50 —

[0112] The polymer of the present invention can be synthesized by, forexample, solution polymerization, precipitation polymerization,suspension polymerization, bulk polymerization or emulsionpolymerization. The polymerization may be initiated by a method of usinga radical initiator or a method of irradiating light or radiant. Thesepolymerizations methods and polymerization initiating methods aredescribed, for example, in T. Tsuruta, Kobunshi Gosei Ho'ho, kaitei-ban(Polymer Synthesis Method, revised edition), Nikkan Kogyo Shinbun Sha(1971) and T. O'tsu and M. Kinoshita, Kobunshi Gosei no Jikken-Ho(Experimental Method of Polymer Synthesis), pp. 124-154, Kagaku Dojin(1972).

[0113] Of those polymerization methods, solution polymerization using aradical initiator is particularly preferred. Examples of the solvent foruse in the solution polymerization include various organic solvents suchas ethyl acetate, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,acetone, dioxane, N,N-dimethylformamide, N,N-dimethylacetamide, benzene,toluene, acetonitrile, methylene chloride, chloroform anddichloroethane. These solvents may be used individually or incombination of two or more thereof or may be used as a mixed solventwith water.

[0114] The polymerization temperature is necessary to be selected inrelation with the molecular weight of the polymer produced or the kindof initiator. The polymerization temperature may be from 0° to 100° C.but it is usually from 30° to 100° C. In the present invention, thepolymerization is preferably performed at from 30° to 60° C. because themonomer for forming the site of A in formula (1) may decompose at a hightemperature.

[0115] Preferred examples of the radical initiator used for thepolymerization include aza-based initiators such as2,2′-azobisisobutyronitile,2,2′-azobis(2,4-dimethylvaleronitrile),2,2′-azobis(2-amidinopropane)dihydrochloride and4,4′-azobis(4-cyanopentanoic acid) and peroxide-based initiators such asbenzoyl peroxide, t-butyl hydroperoxide and potassium persulfate (whichmay be used as a redox initiator by combining, for example, with sodiumhydrogensulfite). In the present invention, an initiator of giving ahalf-value period of 10 hours at a temperature of 70° C. or less (forexample,2,2′-azobis(2,4-dimethylvaleronitrile),2,2′-azobis(4-methoxy-2,4-dimethylvaleronitrile),dimethyl 2,2′-azobis(2-methyl-propionate) and2,2′-azobis[2-(3,4,5,6-tetrahydropropane)-dihydrochloride]) isparticularly preferred.

[0116] The amount of the polymerization initiator used may be controlledaccording to the polymerizability of the monomer or the molecular weightof the polymer intended but it is preferably from 0.01 to 5.0 mol %based on the monomers.

[0117] In synthesizing the polymer of the present invention, thepolymerization may be performed by mixing monomers for forming A, B andC, placing the mixture in a reaction vessel and then adding an initiatoror by passing through a step of adding dropwise those monomers to apolymerization solvent.

[0118] In the image recording medium of the present invention, thepolymer used in the image forming layer is a polymer having both apartial structure having a function of generating an acid by the actionof an acid and breeding an acid by the action of the acid generated fromthe moiety itself and a partial structure of causing change in theabsorption region of from 360 to 900 nm by the action of an acid. Assuch a polymer, a polymer of a single monomer having these functions atthe same time may be used, however, from the synthetic standpoint, astructure represented by formula (6) is generally used.

[0119] The image recording medium of the present invention is generallymanufactured by coating a polymer represented by formula (6) on asupport. In the case where the polymer of the present invention has alsoa function of generating an acid by heat, there occurs change in theabsorption region only by the action of heat and accordingly, thethermosensitive recording material as such used exhibits highsensitivity and good image sharpness. Furthermore, the image recordingmedium of the present invention is advantageous in that the filmthickness can be reduced, a fairly sharp image can be obtained andablation is difficult to take place.

[0120] In the present invention, a slight amount of base may be addedfor the purpose of increasing the storage stability of the imagerecording medium, a compound capable of generating an acid by the actionof light or heat may be separately added so as to increase thesensitivity, or if desired, additives such as a pigment, an antistaticagent and a sticking inhibitor may be added. Furthermore, an overcoatlayer may be provided in order to protect the image forming layer or abackcoat layer may be provided on the back surface of the support. Inaddition, various known techniques for the thermosensitive material maybe used, for example, a single or plurality of undercoat layerscomprising a pigment or resin may be provided between the imagerecording layer and the support.

[0121] In the case of adding a base, an organic base is preferred andpreferred examples thereof include guanidine derivatives (e.g.,1,3-diphenylguanidine, 1,3-dimethyl-guanidine, 1,3-dibutylguanidine,1-benzylguanidine, 1,1,3,3-tetramethylguanidine), aniline derivatives(e.g., aniline, p-t-butylaniline, N,N′-dimethylaniline,N,N′-dibutylaniline, triphenylamine), alkylamine derivatives (e.g.,tributylamine, octylamine, laurylamine, benzylamine, dibenzylamine) andheterocyclic compounds (e.g., N,N′-dimethylaminopyridine,1,8-diazabicyclo[ 5.4.0]-undecene, triphenylimidazole, rutidine,2-picolin). The base is preferably added in an amount of from 1 to 50mol %, more preferably from 5 to 20 mol %, based on the compositionrepresented by A of formula (1).

[0122] The polymer represented by formula (6) may contain a compoundcapable of generating an acid by the action of light or heat andspecific examples thereof are as described above.

[0123] The compound is preferably added in an amount of from 1 to 50 mol%, more preferably from 1 to 20 mol %, based on the site of A in thepolymer.

[0124] In the case of adding a pigment, examples thereof includediatomaceous earth, talc, kaolin, calcined kaolin, titanium oxide,silicon oxide, magnesium carbonate, calcium carbonate, aluminumhydroxide and urea-formalin resin.

[0125] Examples of other additives include an ultraviolet absorbent suchas benzophenone-based ultraviolet absorbent and benzotriazole-basedultraviolet absorbent, a head abrasion or sticking inhibitor comprisinga higher fatty acid metal salt such as zinc stearate and calciumstearate, and waxes such as paraffin, paraffin oxide, polyethylene,polyethylene oxide and caster wax, and these may be added if desired.

[0126] Examples of the support for use in the image recording medium ofthe present invention include papers such as wood free paper, barytapaper, coat paper, cast coat paper and synthetic paper, polymer filmsuch as polyethylene, polypropylene, polyethylene terephthalate,polyethylene-2,6-naphtylene dicarboxylate, polyarylene, polyimide,polycarbonate and triacetyl cellulose, glass, metal foil and non-wovenfabric.

[0127] In the case when the image recording medium of the presentinvention is used for forming a transmitting image, for example, as OHPfilm or photomechanical film, a transparent support is used. For thephotomechanical film, a support having a small coefficient of thermalexpansion and good dimensional stability and having no absorption in thephotosensitive region of the PS plate is selected.

[0128] In the formation of an image using the image recording medium ofthe present invention, heating may be performed by a method ofcontacting the medium with a heated block or plate, a method ofcontacting the medium with a heat roller or heat drum, a method ofirradiating a halogen lamp or an infrared or far infrared lamp heater, amethod of imagewise heating the medium by a thermal head of athermosensitive printer or a method of irradiating a laser ray. When theimage recording medium of the present invention is used for the materialrequired to have high resolution, such as photomechanical material, ascan exposure system using a laser ray is preferred. In order to form animage with a smaller heat energy, the thermosensitive recording materialof the present invention may be previously heated at an appropriatetemperature. In the present invention, after the imagewise heatingaccording to the above-described method, the entire surface is heated ata temperature of from 60° to 150° C. (preferably from 60° to 120° C.) sothat the picture image can be amplified. More specifically, in thepolymer of the present invention, the site of A first decomposes by theheating to generate an acid and further, by the action of the acidgenerated, change in the absorption of B is caused. This reactionproceeds within a very short time upon irradiation of a laser ray butthe picture image can be amplified by the after-treatment.

[0129] In the case of forming an image by the irradiation of a laserray, a dye of absorbing light at the wavelength of the laser ray isnecessary to be present so as to convert the laser ray to the heatenergy. Examples of the laser light source include an excimer laser, anargon laser, a helium-neon laser, a semiconductor laser, a glass (YAG)laser, a carbon dioxide gas laser and a dye laser. Of these, ahelium-neon laser, a semiconductor laser and a glass laser are useful inthe present invention. In particular, a semiconductor laser is moreuseful because the device is compact and inexpensive. The semiconductorusually has an oscillation wavelength of from 670 to 830 nm and a dyehaving absorption in this near infrared region is used. Examples of thedye having near infrared absorption include a cyanine dye, a squaryliumdye, a merocyanine dye, an oxonol dye and a phthalocyanine dye. Specificexamples thereof include the materials described in U.S. Pat. Nos.4,973,572, 4,948,777, 4,950,640, 4,950,639, 4,948,776, 4,948,778,4,942,141, 4,952,552, 5,036,040 and 4,912,083.

[0130] In the case when a compound capable of generating an acid bylight is used, a laser is selected according to the absorptioncharacteristics of the photoacid generating agent as means for forming alatent image. For the purpose of spectral sensitization, various dyesmay be added but in this case, the laser used must be selected takingaccount of the absorption wavelength of the dye.

[0131] The present invention is described in greater detail below byreferring to the Examples, however, it should be understood that thepresent invention is not limited thereto.

[0132] The compounds shown below were dissolved in chloroform and thesolutions obtained each was coated on a 100 μm-thick polyethyleneterephthalate film and dried to obtain a transparent thermosensitiverecording sheet. The polystyrene used was polystyrene bead (diameter:about 3.2 mm) produced by Wako Junyaku KK. Sample 1: E-(35) 0.4 mmol/m²F-(1) 4 mmol/m² H-(1) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85 g/m²Sample 2: F-(1) 4 mmol/m² H-(2) 2 mmol/m² IR dye 113 mg/m² Polystyrene0.85 g/m² Sample 3: E-(35) 0.4 mmol/m² F-(21)/ 4 mmol/m² H-(1) 2 mmol m²IR dye 113 mg/m² Polystyrene 0.85 g/m² Sample 4: F-(35) 0.4 mmol/m²F-(36) 4 mmol/m² H-(1) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85 g/m²Sample 5: E-(35) 0.4 mmol/m² F-(51) 4 mmol/m² H-(1) 2 mmol/m² IR dye 113mg/m² Polystyrene 0.85 g/m² Sample 6: E-(31) 0.4 mmol/m² F-(1) 4 mmol/m²H-(1) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85 g/m² Sample 7: F-(8) 4mmol/m² H-(1) 2 mmol/m² IR dye 113 mg/m² Sample 8: E-(35) 0.4 mmol/m²F-(22) 4 mmol/m² G-(30) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85 g/m²Sample 9: E-(35) 0.4 mmol/m² F-(22) 4 mmol/m² G-(25) 2 mmol/m² IR dye113 mg/m² Polystyrene 0.85 g/m² Sample 10: E-(35) 0.4 mmol/m² F-(22) 4mmol/m² H-(35) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85 g/m²Reference Sample 1: p-Toluenesulfonic acid 4 mmol/m² H-(1) 2 mmol/m² IRdye 113 mg/m² Polystyrene 0.85 g/m² Reference Sample 2: H-(1) 2 mmol/m²IR dye 113 mg/m² Polystyrene 0.85 g/m² Reference Sample 3: E-(31) 0.4mmol/m² Reference Compound 1 4 mmol/m² H-(1) 2 mmol/m² IR dye 113 mg/m²Polystyrene 0.85 g/m² Reference Sample 4: E-(35) 0.4 mmol/m² ReferenceCompound 1 4 mmol/m² H-(1) 2 mmol/m² IR dye 113 mg/m² Polystyrene 0.85g/m² Reference Sample 5: Nitrocellulose (viscosity: 1,000 sec, producedby Dicel Kogyo KK) 0.85 g/m² Reference Dye 1 0.35 g/m² Reference Dye 20.55 g/m² IR Dye 1 113 mg/m² Reference Sample 6: Polyvinyl butyral 0.85g/m² Reference Dye 1 0.35 g/m² Reference Dye 2 0.55 g/m² IR Dye 1 113mg/m² IR Dye:

Reference Compound 1: Reference Dye 1:

Reference Dye 2:

EXAMPLE 1

[0133] Laser Exposure Condition for Image Recording:

[0134] Wavelengths of 8 lines of Spectra Diode Labs No. SDL-2430(wavelength range: 800 to 830 nm) were combined and the output was setat 400 mW to prepare a laser for image writing.

[0135] Each of the samples prepared above was exposed using this laserby setting the beam system at 160 μm, the laser scanning speed at 0.5m/sec (scan center part), the sample feeding speed at 15 mm/sec and thescanning pitch at 8 lines/mm so as to obtain an image of 22 mm×9 mm. Atthis time, the laser energy density on the sample was 5 mJ/mm².

[0136] After Samples 1 to 5 and Reference Samples 1 and 2 were scanexposed under the above-described laser exposure conditions, the colordensity at 360 nm in the exposed area was examined. Thereafter, eachsample after the laser exposure was heat developed under the heatingconditions of 120° C. and 30 seconds and then, the color density in theexposed area and unexposed area was determined.

[0137] The results obtained are shown in Table 1 (the color densityshown is a value corrected by subtracting the absorption density (about0.2) of the raw film). TABLE 1 Color Density Laser Laser Exposed Area (5mJ/mm²) Unexposed Area Heating Heating No Heating (120° C., 30 sec)(120° C., 30 sec) Sample 1 0.8 2.8 0 Sample 2 0.3 2.5 0 Sample 3 0.2 2.60 Sample 4 0.3 2.3 0 Sample 5 0.1 2.1 0 Reference 1.9 2.8 2.8 Sample 1Reference 0 0 0 Sample 2

[0138] It is seen from Table 1 that Reference Sample 2 free of an acidwas not colored at all by the laser exposure or the heat developmentafter the exposure, whereas Samples 1 to 5 of the present inventionexhibited good coloring property at the heat development after the laserexposure. Furthermore, it is seen that in the samples of the presentinvention, the color density in the exposed area increased by the heatdevelopment after the laser exposure but since the unexposed area wasnot colored at all, a high contrast image could be obtained. In Sample2, B-(1) has a function as an acid breeding agent and a function as athermal acid generating agent at the same time and accordingly, a colorimage could be obtained even without separately adding an acidgenerating agent as done in Sample 1.

EXAMPLE 2

[0139] After Samples 1 to 6 and Reference Sample 1 were stored at 60° C.for 3 days, the color density at 360 nm was examined. Thereafter, thesamples each was heat developed under the heating conditions of 120° C.and 30 seconds and then the color density was examined. Furthermore, thecolor density when the samples after storage each was laser exposedunder the above-described exposure conditions and the color density wheneach sample was heat developed (120° C., 30 seconds) after the laserexposure were examined. The results obtained are shown in Table 2. TABLE2 60° C., 3 days No Laser Exposure Laser Exposure (5 mJ/mm²) No HeatingNo Heating Heating (120° C./30 sec) Heating (120° C./30 sec) Sample 1 00 0.7 2.7 Sample 2 0 0 0.4 2.8 Sample 3 0.1 0.2 0.3 2.8 Sample 4 0 0.10.2 2.4 Sample 5 0 0 0.1 2.3 Sample 6 0.5 1.3 0.9 2.7 Reference 0 0 0 0Sample 1

[0140] It is seen from Table 2 that Reference Sample 1 was colored underthe above-described storage conditions, whereas Samples 1 to 5 of thepresent invention were completely free of coloring after the storage andeven after the heat development only was performed, were not colored atall, thus, revealed to have high storage stability. Furthermore, it isseen that Samples 1 to 5 of the present invention exhibited excellentcoloring property at the laser exposure after the storage and at theheat development after the exposure, the same as the results shown inTable 1. It is also seen that Samples 1 to 5 of the present inventionusing a sulfonic acid precursor as the thermal acid generating agent aresuperior in the storage stability as compared with Sample 6 of thepresent invention using an extra acid precursor.

EXAMPLE 3

[0141] Laser Exposure Conditions for Image Recording:

[0142] In the laser exposure method described in Example 1, the energydensity was varied as shown in Table 3 by changing the laser scanningspeed or the laser output.

[0143] Comparison of Image Formation Efficiency:

[0144] Samples 1 to 10 of the present invention and Reference Samples 3and 4 each was exposed by laser scanning and then heat developed underthe heating conditions of 120° C. and 30 seconds. Then, the maximumcolor density at the laser scan center part (image area) was determinedand by comparing the value obtained with the theoretical value in thecase of 100% coloring, the image formation efficiency (coloringefficiency) was calculated. With respect to Reference Samples 4 and 5,the image formation efficiency (decolorization efficiency) wascalculated from the comparison between the density of the laser scancenter part (image area) and the density of the non-image area. Theresults obtained are shown in Table 3. TABLE 3 Image FormationEfficiency (Laser Exposure + Heat Development) Laser Energy Density(mJ/mm²) 10 5 3 Sample 1 89%  97% 85% (Ivention) Sample 2 87%  92% 78%(Ivention) Sample 3 92%  98% 93% (Ivention) Sample 4 90%  89% 68%(Ivention) Sample 5 93%  87% 63% (Invention) Sample 6 96% 100% 97%(Invention) Sample 7 94%  98% 92% (Invention) Sample 8 87% 68% 33%(Invention) Sample 9 98%  93% 48% (Invention) Sample 10 92%  89%  32%(Invention) Reference 72%  13%  0% Sample 3 Reference  2%  0%  0% Sample4 Reference 70%  35%  0% Sample 5 Reference 25%  0%  0% Sample 6

[0145] In the case where the laser energy density was 10 mJ/mm², it wasconfirmed by the observation of the exposed area through an opticalmicroscope that in all samples, ablation was generated at the laserscanning center part. Due to this, the apparent coloring efficiency ofsamples of the present invention which are a coloring type was evaluatedlower than the actual coloring efficiency. On the other hand, in thecase where the laser energy density was 5 mJ/mm² or less, ablation wasnot observed except for Reference Sample 4. It is seen from Table 3 thatout of the samples of the present invention, samples using a dye capableof absorbing light at the wavelength of the laser ray could form animage efficiently even with a low energy (5 mJ/mm² or less) on the orderof not causing ablation and were revealed to have higher sensitivitythan the samples using ablation by the laser ray.

[0146] Furthermore, Samples 1 to 7 had higher sensitivity than ReferenceSamples 3 and 4 using a secondary acid generating agent (ReferenceCompound 1) described in WO94/10606. In particular, Reference Sample 4using a combination of Reference Compound 1 and an acid generating agentof generating a sulfonic acid was scarcely colored.

[0147] Representative synthesis examples of the copolymer of the presentinvention are described below.

SYNTHESIS EXAMPLE 1

[0148] Synthesis of P-(1):

[0149] 1) Synthesis of A-(1)

[0150] 18 g of t-butyl 2-methyl-2-(2-hydroxymethyl)aceto-acetatesynthesized according to the method described in JP-A-8-248561, 19.8 gof triethylamine and 2 g of 4-dimethyl-aminopyridine were dissolved in90 ml of methylene chloride. To the resulting solution, 18 g ofp-vinylbenzenesulfonyl chloride (synthesized by allowing thionylchloride to act on sodium p-vinylbenzenesulfonate) was added, and thesolution was stirred at room temperature for 4 hours. To the reactionsolution 100 ml of water was added, and then the organic layer wasextracted and washed twice with 100 ml of water. The organic layer wasdried over magnesium sulfate and after added thereto 2 mg ofhydroquinone monomethyl ether, concentrated under reduced pressure. Theoily product obtained was purified by a silica gel column chromatography(eluent: n-hexane/ethyl acetate=3/1) to obtain 17.7 g (yield 54.1%) of acolorless transparent oil. The structure obtained was identified by themass spectrometry, elemental analysis and ¹H-NMR.

[0151] Mass spectrometry: M⁺=367 Elemental analysis: Calculated: C:58.68%, H: 6.57%, S: 8.70% Found: C: 58.69%, H: 6.59%, S: 8.71% ¹H-NMR(CDCl₃) δ (ppm): 1.40 (s, 3H), 1.42 (s, 9H), 2.15 (s, 3H), 4.30 (Abq,2H), 5.50 (d, 1H), 5.93 (d, 1H), 6.78 (dd, 1H), 7.58 (d, 2H), 7.85 (d,2H)

[0152]¹H-NMR (CDCl₃) δ (ppm):

[0153] 1.40 (s, 3H), 1.42 (s, 9H), 2.15 (s, 3H), 4.30 (Abq, 2H), 5.50(d, 1H), 5.93 (d, 1H), 6.78 (dd, 1H), 7.58 (d, 2H), 7.85 (d, 2H)

[0154] 2) Synthesis of B-(1)

[0155] 50 g of 4-(2-methacryloyloxyethyl)-2-(2H-benzotriazol-2-yl)phenol(produced by Otsuka Kagaku KK) was dissolved in 300 ml of THF andthereto, 17.4 g of potassium t-butoxy was added under ice cooling. Tothe resulting solution, 50.4 g of di-t-butyl dicarbonate was added, andthe mixture was stirred for 1.5 hours under ice cooling. To the solutionobtained, 300 ml of water was added and further 500 ml of ethyl acetatewas added to extract the organic layer. The organic layer was washedtwice with water, dried over magnesium sulfate, and after adding thereto10 mg of hydroquinone monomethyl ether, concentrated under reducedpressure. The concentrate obtained was recrystallized from acetonitrileto obtain 54 g (yield: 82.3%) of white crystals. The structure wasidentified by the mass spectrometry, elemental analysis and ¹H-NMR.

[0156] Mass spectrometry: M⁺=422 Elemental analysis for C₂₃H₂₅N₃O₅:Calculated: C: 65.23%, H: 5.95%, N: 9.92% Found: C: 65.31%, H: 5.97%, N:9.90% Melting point: 125-127° C. ¹H-NMR (CDCl₃) δ (ppm): 1.51 (s, 9H),1.95 (s, 3H), 3.11 (t, 2H), 4.45 (t, 2H), 5.55 (s, 1H), 6.10 (s, 1H),7.27-7.45 (m, 4H), 7.90 (d, 1H), 7.92 (d, 1H), 8.10 (s, 1H)

[0157] Melting point: 125°-127° C.

[0158]¹H-NMR (CDCl₃) δ (ppm):

[0159] 1.51 (s, 9H), 1.95 (s, 3H), 3.11 (t, 2H), 4.45 (t, 2H), 5.55 (s,1H), 6.10 (s, 1H), 7.27-7.45 (m, 4H), 7.90 (d, 1H), 7.92 (d, 1H), 8.10(s, 1H)

[0160] 3) Synthesis of P-(1)

[0161] 20 g of A-(1) and 11.5 g of B-(1) were dissolved in 50 ml ofbenzene and thereto 202.4 mg of 2,2′-azobis(2,4-dimethylvaleronitrile),followed by stirring under heating at 60° C. for 10 hours. The reactionsolution was added to 500 ml of MeOH and the white polymer precipitatedwas isolated. As a result, 14.5 g (yield: 46.03%) of a polymer (P-(1))having a molecular weight of 5,200 and a molecular weight distribution(Mw/Mn) of 3.7 was obtained. This polymer was identified to be acopolymer of A-(1) and B-(1) by ¹H-NMR and x:y was verified to be 67:33by the elemental analysis. The thermal decomposition temperature wasfrom 145° to 148° C.

SYNTHESIS EXAMPLE 2

[0162] Synthesis of P-(14):

[0163] 2 g of A-(1) and 1.52 g of B-(41) were dissolved in 5 ml ofbenzene and thereto 20.2 mg of 2,2′-azobis(2,4-dimethylvaleronitrile)was added, followed by stirring at 60° C. for 10 hours. The reactionsolution was added to 30 ml of methanol and then white amorphous wasobtained. The supernatant was removed by decantation and the polymer wasdissolved in a slight amount of methylene chloride and added to methanolto cause reprecipitation. The supernatant was removed and the residuewas dried to obtain 220 mg (yield: 6.25%) of a polymer (P-(14)) having amolecular weight of 3,800 and a molecular weight distribution (Mw/Mn) of3.2 was obtained. This polymer was identified to be a copolymer of A-(1)and B-(41) by ¹H-NMR and x:y was verified to be 67:33 by the elementalanalysis. The melting point was from 120° to 123° C., the thermaldecomposition temperature was from 145° to 148° C. and formation of adark green color was confirmed at the same time with the decomposition.

EXAMPLE 4

[0164] The following compounds were dissolved in chloroform and thesolutions obtained each was coated on a 100 μm-thick polyethyleneterephthalate film and dried to prepare a transparent thermosensitiverecording sheet. Sample 1′: P-(1) benzotriazole moiety: 2 mmol/m² IR dye113 mg/m² Reference Sample 1′: p-Toluenesulfonic acid 4 mmol/m²Comparative compound benzotriazole: 2 mmol/m² IR dye 113 mg/m² ReferenceSample 2′: Comparative compound benzotriazole moiety: 2 mmol/m² IR dye113 mg/m² Comparative Compound:

IR Dye (1):

[0165] Laser Exposure condition for Image Recording:

[0166] Wavelengths of 8 lines of Spectra Diode Labs No. SDL-2430(wavelength range: 800 to 830 nm) were combined and the output was setat 400 mW to prepare a laser for image writing.

[0167] Each of the samples prepared above was exposed using this laserby setting the beam system at 160 μm, the laser scanning speed at 0.5m/sec (scan center part), the sample feeding speed at 15 mm/sec and thescanning pitch at 8 lines/mm so as to obtain an image of 22 mm×9 mm. Atthis time, the laser energy density on the sample was 5 mJ/mm².

[0168] After Sample 1′ and Reference Samples 1′ and 2′ were scan exposedunder the above-described laser exposure conditions, the color densityat 360 nm in the exposed area was examined. Thereafter, each sampleafter the laser exposure was heat developed under the heating conditionsof 120° C. and 60° seconds and then, the color density in the exposedarea and unexposed area was determined.

[0169] The results obtained are shown in Table 4 (the color densityshown is a value corrected by subtracting the absorption density (about0.2) of the raw film). TABLE 4 Color Density Laser Laser Exposed Area (5mJ/mm²) Unexposed Area Heating Heating No Heating (120° C., 60 sec)(120° C., 60 sec) Sample 1′ 0.8 2.3 0 Reference 1.2 2.3 2.3 Sample 1′Reference 0 0 0 Sample 2′

[0170] It is seen from Table 4 that Reference Sample 2′ was not coloredat all by the laser exposure or the heat development after the exposure,whereas Sample 1′ of the present invention exhibited coloring propertyequal to or superior to the coloring property of Reference Sample 1′,only the by laser exposure. Furthermore, it is seen that in the Sample1′ of the present invention, the color density in the exposure areaincreased by the heat development after the laser exposure but since theunexposed area was not colored at all, a high contrast image could beobtained.

EXAMPLE 5

[0171] After Sample 1′ and Reference Sample 1′ were stored at 60° C. and70% RH for 3 days, the color density at 360 nm was examined. Thereafter,the samples each was heat developed under the heating conditions of 120°C. and 60 seconds and then the color density was examined. Furthermore,the color density when the samples after storage each was laser exposedunder the above-described exposure conditions and the color density wheneach sample was heat developed (120° C., 60 seconds) after the laserexposure were examined. The results obtained are shown in Table 5. TABLE5 60° C. and 70% RH for 3 days No Laser Exposure Laser Exposure (5mJ/mm²) No Heating No Heating Heating (120° C., 60 sec) Heating (120°C., 60 sec) Sample 1′ 0 0 1.8 2.3 Reference 2.1 2.3 2.3 2.3 Sample 1′

[0172] It is seen from Table 5 that Reference Sample 1′ was coloredunder the above-described storage conditions, whereas Sample 1′ of thepresent invention was completely free of coloring after the storage andeven after the heat development only was performed, was not colored atall, thus, revealed to have high storage stability. Furthermore, it isseen that Sample 1′ of the present invention exhibited excellentcoloring property at the laser exposure after the storage and at theheat development after the exposure, the same as the results shown inTable 4.

EXAMPLE 6

[0173] The following compounds were dissolved in chloroform and thesolutions obtained each was coated on a 100 μm-thick polyethyleneterephthalate film and dried to prepare a transparent thermosensitiverecording sheet. Samp1e 1′: P-(1) benzotriazole moiety: 2 mmol/m² IR Dye(2) 40 mg/m² Sample 2′: P-(2) benzotriazole moiety: 2 mmol/m² IR Dye (2)40 mg/m² Sample 3′: P-(1) benzotriazole moiety: 4 mmol/m² IR Dye (2) 50mg/m² IR Dye (2):

[0174] Laser Exposure Condition for Image Recording:

[0175] The energy density was varied as shown in Table 4 by changing thelaser scan speed or the laser output in the laser exposure conditionsdescribed in Example 4.

[0176] Evaluation of Image Formation Efficiency:

[0177] Samples 1′ to 3′ of the present invention was heat developedunder the heating conditions of 120° C. and 60 seconds after the laserexposure. Then, the maximum color density at the laser scan center part(image area) was determined and by comparing the value obtained with thetheoretical value in the case of 100% coloring, the image formationefficiency (coloring efficiency) was calculated. The results obtainedare shown in Table 6. TABLE 6 Color Density Laser Exposure + Heat LaserExposure Development (3.5 mJ/mm²) (3.5 mJ/mm² + 120° C./60 sec) Sample1′ 1.4 2.3 Sample 2′ 1.3 2.3 Sample 3′ 2.5 4.6

[0178] It is apparent from Table 6 that the compounds of the presentinvention were sufficiently colored even without adding a thermal acidgenerating agent and the polymer itself had a function as a thermal acidgenerating agent.

EXAMPLE 7

[0179] The compounds shown below were dissolved in chloroform and thesolutions obtained each was coated on a 100 μm-thick polyethyleneterephthalate film and dried to obtain a transparent thermosensitiverecording sheet. Sample 2′: P-(2) benzotriazole moiety: 3 mmol/m² IR dye113 mg/m² Sample 3′: P-(3) benzotriazole moiety: 2 mmol/m² IR dye 113mg/m² Sample 4′: P-(7) benzotriazole moiety: 2 mmol/m² IR dye 113 mg/m²Sample 5′: P-(9) benzotriazole moiety: 2 mmol/m² IR dye 113 mg/m² Sample6′: P-(13) leuco dye moiety: 2 mmol/m² IR dye 113 mg/m² Sample 7′:P-(14) leuco dye moiety: 2 mmol/m² IR dye 113 mg/m² Sample 8′: P-(16) 2mmol/m² IR dye 113 mg/m² Sample 9′: P-(21) 2 mmol/m² IR dye 113 mg/m²Sample 10′: P-(22) benzotriazole moiety: 2 mmol/m² IR dye 113 mg/m²Sample 11′: P-(1) benzotriazole moiety: 2 mmol/m² Thermal acidgenerating agent 0.1 mmol/m² IR dye 113 mg/m² Sample 12′: P-(29)benzotriazole moiety: 2 mmol/m² Thermal acid generating agent 0.1mmol/m² IR dye 113 mg/m² Sample 13′: P-(47) benzotriazole moiety: 2mmol/m² Thermal acid generating agent 0.1 mmol/m² IR dye 113 mg/m²Reference Sample 3′: Nitrocellulose 0.85 g/m² (viscosity: 1,000 sec,produced by Dicel Kogyo KK) Reference Dye 1 0.35 g/m² Reference Dye 20.55 g/m² IR Dye 1 113 mg/m² Reference Dye 1:

Reference Dye 2:

Thermal Acid Generating Agent:

[0180] Laser Exposure Condition for Image Recording:

[0181] The energy density was varied as shown in Table 7 by changing thelaser scan speed or the laser output in the laser exposure conditionsdescribed in Example 4.

[0182] Comparison of Image Formation Efficiency:

[0183] Samples 1′ to 3′ of the present invention was heat developedunder the heating conditions of 120° C. and 60 seconds after the laserexposure. Then, the maximum color density at the laser scan center part(image area) was determined and by comparing the value obtained with thetheoretical value in the case of 100% coloring, the image formationefficiency (coloring efficiency) was calculated. With respect toReference Example 3′, the image formation efficiency (decolorizationefficiency) was calculated from the comparison between the density ofthe laser scan center part (image area) and the density of the non-imagearea. The results obtained are shown in Table 7. TABLE 7 Image FormationEfficiency (Laser Exposure + Heat Development) Laser Energy Density(mJ/mm²⁾ 5 3 Sample 1 (Invention)  99% 92% Sample 2 (Invention) 100% 99%Sample 3 (Invention)  93% 82% Sample 4 (Invention)  98% 96% Sample 5(Invention)  97% 90% Sample 6 (Invention)  89% 58% Sample 7 (Invention) 98% 67% Sample 8 (Invention)  99% 92% Sample 9 (Invention)  79% 63%Sample 10 (Invention)  97% 88% Sample 11 (Invention)  97% 96% Sample 12(Invention)  60% 30% Sample 13 (Invention)  43% 25% Reference Sample 3 35%  8%

[0184] It is seen from Table 7 that the samples of the present inventioncould form efficiently an image with a low energy of 5 mJ/mm² and weresuperior to Reference Sample 3′ using ablation by a laser ray.

[0185] While the invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

What is claimed is:
 1. An image recording medium comprising: an acidgenerating agent capable of generating an acid by the action of heat oran acid, which is represented by the following formula (1); and acompound of causing variation in the absorption region of from 360 to900 nm by the intramolecular or intermolecular reaction triggered by theaction of an acid: W¹OP   (1) wherein W¹ represents a residue of an acidrepresented by W¹OH, and P represents an acid-sensitive substituentcapable of splitting off at a temperature of 150° C. or less due tocatalysis by W¹OH.
 2. The image recording medium as claimed in claim 1 ,wherein the acid generating agent represented by formula (1) is acompound selected from the compounds represented by the followingformulae (2) to (5):

wherein R¹ represents an alkyl group or an aryl group; R² represents analkyl group; R³ represents a secondary or tertiary alkyl group having ahydrogen atom at the β-position; and W¹ represents a residue of an acidrepresented by W¹OH;

wherein R⁴, R⁵, R⁶ and R⁹ each represents a hydrogen atom, an alkylgroup or an aryl group; R⁷ and R⁸ each represents an alkyl group, anaryl group or a silyl group; R⁷ and R⁸ may form a ring; X represents Oor S; and W¹ represents a residue of an acid represented by W¹OH;

wherein R¹⁰, R¹¹ and R¹² each represents a hydrogen atom, an alkyl groupor an aryl group; and W¹ represents a residue of an acid represented byW¹OH;

wherein R¹³ represents an alkyl group or an aryl group; and W¹represents a residue of an acid represented by W¹OH.
 3. The imagerecording medium as claimed in claim 1 , which further comprises acompound capable of generating an acid by the action of light or heat inaddition to the acid generating agent represented by formula (1).
 4. Theimage recording medium as claimed in claim 3 , wherein the acidgenerating agent capable of generating an acid by the action of light orheat is a compound capable of generating a sulfonic acid, a carboxylicacid or a phosphoric acid.
 5. The image recording medium as claimed inclaim 1 , wherein the compound of causing variation in the absorptionregion of from 360 to 900 nm by the action of an acid is a compoundhaving at least one amino group substituted by a substituent which isaccelerated to split off by the action of an acid, and causing variationin the absorption region due to the splitting off of said substituent.6. The image recording medium as claimed in claim 5 , wherein saidsubstituent of the amino group, which is accelerated to split off by theaction of an acid, is a secondary or tertiary alkoxycarbonyl grouphaving a hydrogen atom at the β-position.
 7. The image recording mediumas claimed in claim 1 , wherein the compound of causing variation in theabsorption region of from 360 to 900 nm by the action of an acid is acompound having at least one hydroxyl group substituted by a substituentwhich is accelerated to split off by the action of an acid, and causingvariation in the absorption region due to removal of said substituent.8. The image recording medium as claimed in claim 7 , wherein thesubstituent of the hydroxyl group, which is accelerated to split off bythe action of an acid, is a secondary or tertiary alkoxycarbonyl grouphaving a hydrogen atom at the β-position, an alkoxymethyl group or asilyl group.
 9. The image recording medium as claimed in claim 1 , whichfurther comprises an infrared ray absorptive substance.
 10. A method forrecording an image, which comprises scan-exposing the image recordingmedium claimed in claim 1 by a laser beam and then heating the entiresurface of the image recording medium at a temperature of from 60° to150° C.
 11. A polymer represented by the following formula (6):

wherein A represents a repeating unit obtained by polymerizing at leastone vinyl monomer having a function of generating an acid by the actionof an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.
 12. The polymer as claimed inclaim 11 , wherein A in formula (6) has a function of generating an acidby the action of heat.
 13. The polymer as claimed in claim 12 , whereinA in formula (6) is a repeating unit obtained by polymerizing at leastone vinyl monomer represented by the following formula (7):

wherein R²¹ represents an electron-withdrawing group having a Hammett'sup value greater than 0; R²² represents an alkyl group or an aryl group;R²³ represents a secondary or tertiary alkyl group having a hydrogenatom at the β-position; W² represents a residue of an acid representedby W²OH; and any one of the substituents R²¹, R²², R²³ and W² representsa polymerizable vinyl group.
 14. An image recording medium whichcomprises a polymer having both (i) a partial structure of generating anacid by the action of an acid and further breading an acid by the actionof an acid generated from the partial structure itself and (ii) apartial structure of causing variation in the absorption region of from360 to 900 nm by the action of an acid.
 15. The image recording mediumas claimed in claim 14 , wherein the polymer is a polymer represented byformula (6):

wherein A represents a repeating unit obtained by polymerizing at leastone vinyl monomer having a function of generating an acid by the actionof an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.
 16. The image recording medium asclaimed in claim 14 , which further comprises an acid generating agentcapable of generating an acid by the action of light or heat.
 17. Theimage recording medium as claimed in claim 14 , which further comprises(i) an acid generating agent capable of generating an acid by the actionof heat and (ii) an infrared ray absorptive substance.
 18. The imagerecording medium as claimed in claim 14 , wherein the polymer is apolymer represented by formula (6):

wherein A represents a repeating unit obtained by polymerizing at leastone vinyl monomer having a function of generating an acid by the actionof an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100, whererin the image recordingmedium further comprises an infrared ray absorptive substance, andwherein A in formula (6) has a function of generating an acid by theaction of heat.
 19. A method for recording an image, which comprisesscan-exposing the image recording medium claimed in claim 14 by a laserbeam.
 20. A method for recording an image, which comprises scan-exposingan image recording medium claimed in claim 14 by a laser beam and thenheating the entire surface of the image recording medium at atemperature of from 60° to 150° C.
 21. An image recording mediumcomprising a polymer having both a moiety of generating an acid by theaction of an acid and a partial structure of causing variation in theabsorption region of from 360 to 900 nm by the action of an acid. 22.The image recording medium as claimed in claim 21 , which furthercomprises an ac-id generating agent capable of generating an acid by theaction of light or heat.
 23. The image recording medium as claimed inclaim 21 , wherein the polymer is a polymer represented by formula (6):

wherein A represents a repeating unit obtained by polymerizing at leastone vinyl monomer having a function of generating an acid by the actionof an acid and further breeding an acid by the action of an acidgenerated from the vinyl monomer itself; B represents a repeating unitobtained by polymerizing at least one vinyl monomer having a partialstructure of causing an absorption region of from 360 to 900 nm by theaction of an acid; C represents a repeating unit obtained bypolymerizing at least one vinyl monomer copolymerizable with A and B;and x, y and z each represents % by weight and satisfy the conditions of1≦x≦99, 1≦y≦99, 0≦z≦98 and x+y+z=100.
 24. The image recording medium asclaimed in claim 21 , wherein A in formula (6) is a repeating unitobtained by polymerizing at least one vinyl monomer represented byformula (7):

wherein R²¹ represents an electron-withdrawing group having a Hammett'sop value greater than 0; R²² represents an alkyl group or an aryl group;R²³ represents a secondary or tertiary alkyl group having a hydrogenatom at the β-position; W² represents a residue of an acid representedby W²OH; and any one of the substituents R²¹, R²², R²³ and W² representsa polymerizable vinyl group.
 25. The image recording medium as claimedin claim 21 , which further comprises an infrared ray absorbingsubstance.
 26. A method for forming an image, which comprisesscan-exposing the image recording medium claimed in claim 21 by a laserbeam.
 27. A method for forming an image, which comprises scan-exposingthe image recording medium claimed in claim 21 by a laser beam and thenheating the entire surface of the image recording medium at atemperature of from 60° to 150° C.